Allow theme developers to add the default favicon filename to the theme's .info.yml

Bartik is no longer there. Using Olivero instead.

Last to do is making a change record like:
https://www.drupal.org/node/2939152 →

and when released adding to documentation:
https://www.drupal.org/docs/develop/theming-drupal/defining-a-theme-with... →

It does need a change record.

But nitpicky can the new functions have a return typehint.

Also for the test unless we are testing olivero specific code we should use stark.

With the test now using stark

Added an issue fork. Used patch from 33.

Then I alter the test to be in sync with the tests used for the logo option test.

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hiding patches

Test failure seems legit to the ticket.

Test is failing because the Stark theme itself has no favicon.ico.

3 options to solve this.

1. Use an other core theme for this test, (only Olivero has a favicon)
2. Add a favicon to the Stark theme.
3. Add a favicon to a test theme and use that.

Okay so lets just switch back to claro. sorry for the noise.

Claro also doesn't have a favicon in its theme root 🙈 So I will use Olivero for that case. Rest of test cases still uses Stark.

Don't understand why the last one fails.

The same logic for testing a logo is not failing

    $theme_installer->install(['test_theme']);
    \Drupal::configFactory()
      ->getEditable('system.theme')
      ->set('default', 'test_theme')
      ->save();
    $theme = $theme_handler->getTheme('test_theme');

    drupal_static_reset('theme_get_setting');

    // Tests logo set in test_theme.info.yml.
    $expected = '/' . $theme->getPath() . '/images/logo2.svg';
    $this->assertEquals($expected, theme_get_setting('logo.url', 'test_theme'));